Unit fuel injectors operated by cams, have long been used in compression ignition internal combustion engines for their accuracy and reliability. The unit injector typically includes an injector body having a nozzle at one end and a cam driven injector plunger mounted for reciprocating movement within the injector body. In the typical unit fuel injector, a mechanical link, which is cam actuated, physically communicates with a lower, intermediate or upper plunger which moves inwardly, during the injection event, to force fuel out of an injector orifice(s) into the combustion chamber. Prior to each injection event, fuel is metered into an injection chamber with the amount of fuel injected being controlled on a cycle by cycle basis.
Internal combustion engines are subjected to a variety of external as well as internal variable conditions ultimately affecting the performance of the engine. Examples of such conditions are engine load, ambient air pressure and temperature, timing, power output and type and amount of fuel being consumed. To achieve optimal engine operation fuel must be injected at a very high pressure to cause the maximum possible atomization of the injected fuel. In addition, the interval of injection needs to be carefully timed during each cycle of injector operation with respect to the movement of the corresponding engine piston.
Attempts have been made to provide independent control over the quantity and timing of injection during each cycle using a collapsible hydraulic link to selectively change the effective length of the cam operated fuel injector plunger assembly. For example, in U.S. Pat. No. 4,281,792 to Sisson et at., a unit fuel injector is disclosed including a two part plunger having a variable volume hydraulic timing chamber separating the plunger sections and a single solenoid valve which commences the injection on the downstroke of the plunger by closing to form a hydraulic link between the plunger sections. On the upstroke, the solenoid valve opens at a selected point to control the quantity of fuel metered below the lower plunger for injection on the subsequent downstroke. Similarly, U.S. Pat. No. 4,531,672 to Smith discloses a unit fuel injector containing a fluid timing circuit and a fluid metering circuit for providing fuel flow to respective timing and metering chambers by means of a single solenoid valve which is adapted to control separately timing and metering through variation in the time of opening and closing, respectively, during each cycle of operation. While these types of injector designs provide adequate control over both timing and metering, both designs use common metering and timing passages thereby requiring engine fuel to be used as the timing fluid. As a result, a greater amount of fuel is supplied to the unit injector than is necessary to supply the injection chamber since fuel is continually cycled through the timing chamber during injector operation. This results in a substantial amount of timing fuel being heated within the injector and subsequently drained or spilled to the fuel supply tank. The hot fuel returned to the supply tank causes undesired fuel evaporation and often requires the installation of fuel cooling heat exchangers to reduce the temperature of the fuel in the supply tank. In addition, since these fuel injector designs use a common fuel supply rail for all the injectors of an engine, a sharp pressure increase or spike is generated in the fuel supply rail each time the timing fuel spills from the high pressure timing chamber of each injector. Consequently, the pressure spike from one injector can adversely affect the reliability and control of injection metering and/or timing in other injectors.
The problems associated with draining excessive quantities of hot fuel to the supply tank and the accompanying pressure spikes have become even more apparent due to recent and upcoming legislation placing strict emission standards on engine manufacturers resulting from a concern to improve fuel economy and reduce emissions. In order for new engines to meet these standards, it is necessary to produce fuel injectors and systems capable of achieving higher injection pressures, shorter injection durations and more accurate control of injection timing. High injection pressures may be achieved in a number of ways such as by varying the cam profile, plunger diameter and/or number and size of injection orifices. Various techniques have been developed to control timing including mechanical, e.g. racks for rotating injector plungers having helical control surfaces; electronic, e.g. valves for controlling the start and/or end of injection and hydraulic, e.g. variable length hydraulic links. With respect to the latter, timing is advanced by introducing more timing fluid into the timing chamber which effectively lengthens the fluid link between the injector plungers. In the typical injector, as a result of this lengthened link, the pumping plunger commences injection and/or reaches its bottom most position at an earlier point in the rotation of the corresponding cam. Accordingly, fuel injection can occur at a point in the combustion cycle when the piston of the engine is still moving upward.
Because fuel is normally used as the timing fluid in injectors of this type, the amount of fuel which is supplied to and drained away from the injector of an engine necessarily increases as compared with injectors employing non-hydraulic timing control or no timing control. The amount of heat absorbed by the fuel and ultimately the temperature of the fuel in the fuel supply tank has been found to increase to an unacceptably high level.
Another problem encountered in fuel injectors of the type disclosed in '792 Sisson et al. and '672 Smith is overpressurization of the injector body during the timing phase of the cycle. As the upper plunger is driven into the timing chamber, timing fuel is forced out of the timing chamber back through the solenoid valve via timing passages into the common supply passages in the injector body. This flow of timing fuel into the supply passages in the injector causes excessive fuel pressure around the solenoid valve and in the injector body. As a result, a relief valve must be incorporated into the spacer portion of the injector to relieve fuel to drain thereby preventing excessive pressure build up in the injector body and possible extrusion of the O-ring seal around the solenoid valve. Moreover, the pressure increase due to pre-injection timing spill back to the fuel supply rail can have deleterious effects on the operation of other injectors. To avoid this problem in current injector designs, the fuel inlet, such as inlet 48 of the '672 Smith injector, formed in the retainer (86 of the '672 Smith injector) is reduced in size to form a starvation orifice and thereby dampen out pressure spikes that would otherwise pass into the supply rail. While useful for their intended purposes, such restricted starvation orifices require the supply rail pressure to be higher in order to provide sufficient fuel metering capability.
Other fuel injector designs which provide for variable timing and metering are disclosed in U.S. Pat. Nos. 4,249,499 to Perr and 4,410,138 to Peters et al. The unit injector design disclosed in the '499 Perr patent includes a timing mechanism having movable pistons connected between a cam drive and an injector plunger that allow timing fluid to enter a timing chamber to form a variable length hydraulic link between the pistons depending on the pressure of the supply wherein the length of the link determines the point at which injection is initiated. The timing fluid circuit, which preferably uses engine lubricant, is separate from the fuel supply or metering circuit. Therefore, since lube oil is used as a timing fluid in a separate timing circuit, neither of the above-mentioned hot fuel drain and pressure spike problems are encountered in this design. However, this design requires a separate control device for both injector timing, in the form of a variable pressure timing fluid mechanism, and for fuel metering in the form of pressure-time metering. Consequently, both timing fluid pressure and metering fuel pressure are critical variables which must be carefully controlled for proper timing and metering.
U.S. Pat. No. 4,410,138 to Peters et al. discloses a fuel injector having infinitely variable timing using a two part injector plunger which forms a variable link timing chamber between the upper and lower plungers for receiving timing fluid. Here again, although the timing fluid circuit is completely separate from the fuel metering circuit, precise control of both the timing fluid pressure and metering fuel pressure are necessary for accurate and reliable control of timing and metering.
Another important concern accentuated by higher injection pressures is the need to adequately cool unit injectors during operation. In the fuel injector designs disclosed in U.S. Pat. Nos. 4,281,791 to Sisson et al. and 4,531,672 to Smith, both the metering fuel and the timing fuel inherently function to cool the unit injector. However, it has been discovered that when fuel is used as the timing fluid, excessive heat may be absorbed by the fuel resulting in the fuel assuming an unacceptably high temperature over extended periods of engine operation. Thus, in order to ensure adequate cooling of the injector, the fuel in the fuel supply tank must be cooled using expensive coolers.
As shown in U.S. Pat. No. 5,072,709 to Long et al., some fuel injectors require one or more biasing springs positioned in the timing chamber to bring the metering plunger to a full and precise stop during the metering phase. Since fuel is used as timing fluid fuel pressure is the same on both sides of the metering plunger. The bias spring creates enough bias pressure to overcome the inertial effects of the motion of the metering plunger to stop the plunger movement during metering. However, the bias spring also creates a fixed preload which must be overcome by fuel pressure above the preload setting in order to move the plunger. This requirement of overcoming the preload of the bias spring is an undesirable feature of the design which becomes particularly emphasized at start up or cranking as the fuel or fluid pressure must be increased to a point above the spring bias preload before adequate fuel metering can commence.
An important requirement of unit fuel injectors using engine fuel as timing fluid is to provide a leak off passage between the uppermost plunger and the rocker arm or driving assembly. Without such a leak off passage, fuel leakage by the uppermost plunger would cause the fuel to be mixed with the engine lubrication oil supplied to the rocker arm and linkage assembly impairing the lubrication qualities of the lube oil and ultimately increasing engine wear.
Consequently, there is a need for a fuel injector which is capable of meeting high injection pressure requirements while adequately cooling the injector internals and which uses a simple and effective timing fluid circuit design to accurately and reliably control both timing and metering of fuel injection without causing excessive heating of the engine fuel.